The present invention relates to a process for the desulfurization of coal in which the inorganic sulfur (pyrite) and the organic sulfur are removed from treated coal ore.
The dependance on fossil fuels, both in the United States and throughout the world as a source for energy and raw materials has created a demand that has inflated the price of those resources that are easily obtained and easily utilized. Alternate fuels which heretofore have been uneconomical or unfeasible to use as energy or raw material sources would be readily accepted if they could be made compatible with the economic and environmental requirements. Specifically, the use of high sulfur coal would be particularly advantageous if economical processes for reducing the sulfur content could be employed allowing the desulfurized coal to be used in the place of the more expensive naturally occurring low sulfur coals.
The high sulfur coals the bulk of the coal reserves that have been located and identified. As an energy resource, it has been predicted that these known reserves would sustain domestic United States requirements for the next 200 years. At present, commercial exploitation of these reserves is severely limited since the combustion of high sulfur coal results in significant air pollution which typically requires costly abatement procedures for use. Even though abatement procedures can be effective at reducing the sulfur dioxide emissions, none are recognized to be 100 percent effective 100 percent of the time. This situation elevates concerns about the continued use of high sulfur coal products, especially with regard to acid rain.
An alternative to the abatement practices employed in the use of high sulfur coals, is to remove the offending sulfur compounds prior to combustion. An economical and feasible process for achieving this would allow the utilization of coal reserves, largely untapped, thereby reducing dependence on alternate fossil fuels, and at the same time reducing the concerns and chances for environmental pollution. The present invention, as will be seen, achieves the dual goals of efficiency and economy that are required to maximize the utilization of the vast reserves of high sulfur coal.
The inclusion of sulfur in coal occurs in two forms primarily. The first is inorganic, or pyritic, which generally occurs as pyrite, FeS.sub.2, and associated compounds. The second is organic sulfur, which is sulfur that is linked to the hydrocarbon composition of the coal itself. The distinction between the two forms of coal-related sulfurs is further evidenced in their respective chemical reactivity. The pyritic form undergoes reactions more consistent with ionic salts. The dominance of the iron in the compound also dictates physical characteristics that may be exploited for purposes of separation. The organic sulfurs, on the other hand, exhibit covalent bonding characteristics more like that of organic molecules. Since the desired end product in the desulfurization of coal is an organic constituent, physical differences between the organic sulfurs and final product cannot be exploited as easily as in the case of the pyritic sulfurs. Thus, it becomes necessary to utilize other chemical properties of the organic sulfur in order to achieve separation yields of any significance.
The physical removal of the pyritic sulfurs from coal has been achieved by a number of means, both physical and chemical. While some processes for pyritic removal are relevant in terms of feasibility and economy, without the co-removal of organic sulfurs, the ultimate utilization of any such process is severely limited. However, successful pyritic removal has occurred through the use of floatation, oil-water separation, magnetic separation, and combinations of thermochemical and magnetic processing to name a few. The utilization of these techniques has resulted in a satisfactory result when the interest is in the removal of the pyritic sulfur content exclusively. As mentioned before, this focus results in limitations in applying the technique to all coals since the organic sulfurs are still resident after treatment.
Attempts to liberate the organic sulfurs from coal have included chemical treatment, as would be expected, particularly with carbon disulfide, and also by application of thermal energies to initiate intramolecular liberalization of sulfur. A two-step process for desulfurization of organic sulfurs is known where the coal is subjected to an oxidizing environment and then subsequently treated with alkaline solutions. These processes all suffer from the necessary input of resources, either chemical or thermal, that cannot be recovered and otherwise impair the economic feasibility of the desulfurization process.
More recently, work has been done utilizing olefins, aldehydes or ketones that effectively react as "sulfur traps" in reactions with organic sulfurs. These reactions occur under pressure which shifts the equilibrium of the trapping reaction towards the sulfurized trapping material. Once the pressure on the separated sulfurized trapping material is reduced and the temperature is elevated, the reaction is reversed to release hydrogen sulfide and regenerate the original trapping reactant. The process takes place under conditions of elevated temperature and pressure and requires batch processing by reactor.
Another recent effort in the removal of organic sulfurs involves the washing of coal particles with an aqueous solution of copper ions. The process proceeds under elevated conditions of temperature and pressure and allows both organic and inorganic sulfurs to be dissolved in the copper solution. After sufficient reaction time has been established, the coal solids are rinsed free of the copper solution thereby separating the solubilized organic and inorganic sulfurs from the remaining coal solids. Both this process and the previously discussed process using olefins, aldehydes and ketones requires the step-wise treatment of coal in reaction vessels. While the recent processes have reduced the cost of chemical inputs, the efficiency is limited by batch type procedures requiring capital intensive equipment.
Despite the obvious incentives for feasible processes for the organic and inorganic desulfurization of coal, no one process has been able to demonstrate efficiency and economic feasibility in the continuous production of low sulfur coal products. The present invention achieves such a result in a manner not related to the processes previously known.